1. Field of the Invention
The present invention relates to a dielectric resonator filter adopted in an receiver for the ultrahigh optical transmission system and a clock extraction device using the same, and more particularly, to an electric filter extracting a clock signal from the received data to reproduce a clock signal in an optical receiver and an open-loop type clock extraction device using the same.
2. Description of the Related Art
Clock extraction devices are slightly differently constituted according to types of used transmission signals. In a case where NRZ signals are used, signals corresponding to clock frequencies do not exist. Thus, the clock extraction devices additionally include nonlinear circuit blocks to generate clock signal components. In a case where RZ signals including components of clock signals are used, the clock extraction devices don't need nonlinear circuit blocks.
Clock extraction devices for optical communication systems require electric filter blocks to extract high-quality clock signals and can be classified into closed-loop structure using phase-locked loops (PLLs) and open-loop structure using passive bandpass filters (BPFs).
The open-loop devices include simpler circuit structures and can be implemented more easily at a transmission speed of Gb/s or more as compared to the closed-loop devices. Also, in the open-loop devices, some different techniques have been adopted for the passive BPFs depending on transmission speeds of systems. SAW filter or tank circuit using lumped R, L, and C has been used at a transmission of Gb/s or less. Disc type dielectric resonator filter, which has high-Q value in microwave frequencies, has been used at a transmission of several Gb/s or more.
However, the disc type dielectric resonator filter has been manufactured in the form of independent module with the input and output coaxial connectors. Thus, the conventional disc type dielectric resonator filter is difficult to integrate with other electrical circuits, such as nonlinear circuits and clock amplifiers, in a clock extraction device. Nonlinear circuit blocks and clock amplifier blocks realized on a microwave substrate have been separately manufactured in the form of individual modules in which the input and output connectors are installed, too.
Accordingly, conventional dielectric resonator filters hinder open-loop clock extraction devices using dielectric resonator filters from being compact. Thus, these configurations using the individual modules increase total connection paths in the open-loop clock extraction devices. As a result, the signal loss increases.
Also, in a case of several Gb/s clock extraction devices using the conventional dielectric resonator filters, a plurality of undesired spurious pass bands exist in the vicinity of central frequencies of the conventional dielectric resonator filters that mean clock frequencies. Thus, additional BPFs are required to remove the undesired spurious pass bands.